A control stick is often used for manually controlling the movement of motor-driven machines or instrument parts whereby the deflection of the control stick determines the speed of the part to be moved. Such control sticks usually include a transmitter with a progressive characteristic which provides that an over-proportional driving action is obtained with full deflection.
Control sticks with speed control are, however, badly suited for fine positioning tasks for reasons among which is the dead point at the center position. For example, the movement along a predetermined path is not easily possible or requires very well trained operating personnel.
Control elements proportional to travel are needed for fine positioning tasks, that is, actuating manipulators having a transmitter which delivers a signal proportional to the actuating displacement. Such a manipulator is known, for example, under the name "track ball" which is primarily an input device for computer equipment such as for controlling marks on display screens.
The last-mentioned displacement-proportional manipulators do permit precise positioning. However, when working with such devices, the condition is disturbing that one must roll the ball a substantial length of time to locate positions which are far apart from each other and the time needed for such rolling action is longer in proportion to the sensitivity at which the step-down of the actuating manipulator is set.
This situation can be alleviated in a manner analogous, for example, to the focusing movement of a microscope by providing the known mechanical combination of a coarse-fine drive with a second manipulator having a lower reduction or a switch to switch the reduction ratio. However, this would require an additional control element and would require an additional manual movement by the operator.